1. Field of the Invention
The present invention relates to a metallic porous sheet and a method of manufacturing the metallic porous sheet, and more particularly, to a method of manufacturing a metallic porous sheet by using a three-dimensional net-shaped porous sheet consisting of a foamed sheet, a nonwoven sheet, a mesh sheet or a plurality of sheets layered one on the other by combining at least two of the above three types of sheets with each other. The metallic porous sheet is used as an electrode plate of a nickel cadmium battery, a nickel hydrogen battery, a lithium battery, a fuel battery or a battery plate for use in a car by charging activating powders into pores of the metallic porous sheet.
2. Description of the Related Arts
The present applicant proposed metallic porous sheets to be used as an electrode plate of a battery. The proposed porous sheet consists of a foamed sheet, a nonwoven sheet, a mesh sheet or a plurality of sheets layered one on the other by combining at least two of the above three types of sheets with each other. The porous sheet is formed into a metallic porous sheet by electroplating (Japanese Laid-Open Patent Publication Nos. 1-290792 and 3-130393).
If the foamed material, the nonwoven sheet or the mesh sheet to be used as the base material of the metallic porous sheet consists of an organic substance such as synthetic resin, natural fiber, cellulose, paper or the like; or an inorganic substance such as glass or the like, it is necessary to impart conductivity to the base material. It is preferable to impart conductivity to the base material even though the base material is made of metal.
As methods of imparting conductivity to the base material, carbon is applied to the surface of the base material or the surface of the porous sheet is coated with a conductive material such as metal by chemical plating or vaporization.
The conventional method of imparting conductivity to the base material has the following problems:
(a) Vaporization
It is necessary to perform vaporization in a vacuum device. It is uneasy to evaporate metal over the porous sheet in the vacuum device while it is being transported continuously along a transport path, because air leakage occurs at the entrance or the exit of the vacuum device. Thus, it is difficult to maintain vacuum state. Further, it is necessary to prepare a large equipment and manufacturing cost becomes high and further, it takes much time to impart conductivity to the porous sheet.
(b) Chemical plating method
A large number of processes is required to perform chemical plating method and a great number of chemicals is required to be managed. In particular, it costs high to prepare a device for treating waste solution. In addition, the chemicals cost high.
(c) Carbon applying method
Although this method is less expensive than vaporization method and chemical plating method, a lot of carbon remains as impurity on the surface of the base material. Further, the electric resistance of a conductive layer formed on the base material is as high as 100-200 .OMEGA./cm. Thus, it is difficult to perform electroplating process at high current. In order to obtain high current, it is necessary to run a line for transporting the base material at a slow speed, for example, as slow as 0.1-0.5 m/minute to prevent the base material from being burnt.
Of the above three methods, carbon applying method is mainly adopted because the execution of this method is less expensive than that of the other two methods.
This method has, however, disadvantage that the electric resistance of the conductive layer is high and thus the line for transporting the base material is traveled at a slow speed.
Accordingly, in order to increase productivity by driving the line for transporting the base material at a high speed, it is necessary to reduce the electric resistance of the conductive layer to less than 30 .OMEGA./cm so as to obtain high current in the electroplating process to be performed subsequently to conductivity-imparting process.
Further, carbon applied to the surface of the base material to impart conductivity thereto is not completely eliminated by burning out of carbon and resin and sintering metal plated on the carbon, but a small amount of carbon remains on the surface of the base material.
In forming the metallic porous sheet by using a base material consisting of a resin sheet, it is necessary for an electroplated layer to have a required metallic amount totally. Therefore, electricity rate becomes high and it takes a long time to electroplate the conductive metallic layer. Thus, the metallic porous sheet cannot be manufactured with a high productivity and at a low cost.